1. Field of the Invention
The present invention relates to a method of driving piles into the ocean floor utilizing a pile-driving hammer; and more particularly, the present invention relates to a method of driving piles in great depths of water using an underwater pile-driving hammer.
2. Description of the Prior Art
In the installation of an offshore structure, or jacket structure, such as a drilling or production platform, it is the general practice to secure the structure in some manner to the ocean floor to prevent overturning of the structure. A conventional technique for securing the offshore structure to the ocean floor is the driving of elongated piles into the ocean floor. Typically, an offshore structure will have several legs to support the structure, with one or more piles being provided at each leg depending upon the type of soil into which the piles are to be driven. Generally, piles will be carried in pile sleeves attached to the outside of the support legs during onshore fabrication of the structure.
Driving of the piles has typically been done by a compressed air or steam hammer that delivers repetitive blows to the top of a pile. In most cases, and particularly when great depths of water are involved, it is necessary to employ a pile follower. A pile follower is an extension that contacts the top of the pile and is struck by the pile hammer. Followers have several disadvantages; they are of extremely heavy weight and are expensive. Greater inefficiency is introduced in the pile driving system due to the believed energy loss in transmitting a blow through the follower to the upper end of the pile. Also, followers are often joined in sections with joints which can fail and require repairs that are both costly and difficult to make. The disadvantages of followers are most acute when it is necessary to drive a pile in a deep body of water. In such a case, it is believed necessary to employ an extremely large pile driving hammer to overcome the energy loss through the follower sections, and a derrick barge must be provided to handle the great weight and length of the follower.
Depending on the depth of water, pile length, required pile penetration into the ocean floor, and the height of the jacket structure, pile followers may consist of a number of sections each one of which must be handled onto and off of each pile. These multiple handling steps are extremely time consuming and expensive.
An alternate approach to pile driving has been to dispose steam and air hammers underwater at the submerged locations on the top of the piles to drive the piles directly. Representative of this approach is the HYDROBLOK.TM. type HBM-3000 pile driver which is manufactured by Hollandsche Beton Mattschappij bv. The HYDROBLOK.TM. system includes a casing having a pile sleeve provided at its lower end to center the hammer onto the pile. The ram and hydraulic power unit for the HYDROBLOK.TM. hammer are contained within the casing which is purged of water by air pressure supplied by an air line extending from the surface of the water. Ballast is provided on the sleeve to assist in holding the hammer down on the pile.
A possible disadvantage of the use of a large air filled casing is the buoyant force which is produced. The HYDROBLOK.TM. hammer is a differential hammer requiring a gravity reaction for the force acting on the ram during its downstroke. Any positive buoyancy created by the casing must be replaced by ballast which increases the dry handling weight of the hammer.
Another major problem encountered is that of initially placing this hammer in contact with a pile disposed in a deep body of water. The diameter of the hammer is several times the diameter of the pile to be driven and, therefore, the hammer cannot be merely dropped into alignment with the top of the pile on a conventional jacket structure. Accordingly, when using this hammer to date, followers are still employed, thus eliminating any time or cost savings which could be derived from utilizing a hammer capable of operating underwater.
Another hammer device that is disclosed as being suitable for operation totally submerged in water is the airgun repeater pile driver disclosed in U.S. Pat. No. 3,817,335 and assigned to Bolt Associates, Inc. Although the patent discloses details of a hammer device and characterizes it as a device capable of underwater operation to drive a pile, no teachings are provided of a suitable method for effective aligning and bringing the hammer into contact with a pile, especially a pile located in deep water.
Another approach to the driving of piles by a hammer device involves the disposition of a pile driving hammer within the leg of an offshore structure, which hammer engages the upper end of the pile to deliver driving blows thereto. The hammer is provided with a hollow case with the ram movable within a casing. The hammer and casing are raised and lowered within the leg by a flexible cable. This arrangement is disclosed in U.S. Pat. No. 3,604,522, issued to S. C. Doughty. A seeming disadvantage and hazard to the system proposed by Doughty is that the construction of the hammer, with its anvil being an integral part of the casing, appears to be likely to have the casing pulled into tension, whereby the casing will become broken or fractured.
Accordingly, a method of driving piles in great depths of water using an underwater pile-driving hammer which would substantially increase the effective hammer efficiency over current methods, reduce the size or eliminate the need of derrick barges required to support the pile driving operations, and reduce the time necessary to install deep water piling is highly desirable.